Repair Systems — 05
Internal Cracking
Technical product reference for flexible crack filler and flexible PU sealant systems for internal plasterboard and render crack repair in Australian Class 2 strata apartment buildings.
What are internal crack repair systems?
Internal cracks in Class 2 strata apartment buildings form for a range of reasons — long-term building settlement, thermal movement in framing and lining materials, moisture cycling in plasterboard and render, and live load deflection of floor and ceiling systems. The critical distinction before any crack repair is whether a crack is cosmetic (dormant, non-structural) or structural. Structural cracks — typically wider than 3mm, showing step displacement, or associated with movement of building elements — must be assessed by a structural engineer before any filling or concealment is undertaken. Crack filling is appropriate only for dormant cosmetic cracks.
Two product types are used for internal crack repair in Australian strata buildings. Acrylic gap fillers — such as Selleys No More Gaps Interior Multipurpose — are water-based, low-odour, and paintable, and are suited to hairline and fine dormant cracks in plasterboard, cornices, and thin render coatings. For wider cracks, active movement joints, or locations in render and masonry where ongoing thermal movement is expected, a 1-part flexible sealant based on silane-terminated polymer technology — such as Sikaflex-111 Stick & Seal — provides greater elongation, better movement accommodation, and is also overpaintable once cured.
Painting over repaired cracks requires the filler or sealant to be fully cured before any coating is applied — premature painting over uncured filler can cause paint adhesion failure and visible surface irregularity. The filled crack must be sanded flush with the surrounding surface, primed if required, and finished with a compatible interior paint system. For movement joints filled with PU sealant, backer rod should be installed to control the sealant depth and prevent three-sided adhesion, which would prevent the joint from accommodating movement.
Do not confuse with:
- Structural crack repair — crack injection or stitching for structural concrete or masonry — not internal cosmetic filler
- Facade crack repair — external crack systems for rendered or masonry facades — not internal cosmetic crack products
- Cornice replacement — damaged or severely cracked cornices may require section replacement, not just filling
Product Reference
2 products — 2 brands — internal crack repair systems — scroll to view all
Selleys No More Gaps Interior Multipurpose
Acrylic gap filler for internal cracks
System Description
Selleys No More Gaps Interior Multipurpose is a water-based acrylic gap filler widely used in Australian residential and strata apartment buildings for filling dormant hairline and fine cracks in plasterboard, render, cornices, and internal joints. Water-based formulation — low odour, easy clean-up with water, and paintable once fully cured with standard interior paint systems. Suitable for movement-related hairline cracks at cornice junctions, butt joints in plasterboard, and fine render surface cracks. Not suitable for active (moving) cracks, wet area applications, or cracks wider than approximately 5mm without backing. Crack must be dormant — active cracks will re-crack through the filler. For movement joints and wider cracks, use a PU sealant with backer rod.
Technical Properties
- Water-based flexible acrylic — low odour, easy water clean-up during application
- Paintable once fully cured with standard interior acrylic paint systems
- Suitable for hairline to fine cracks in plasterboard, render, and cornices
Limitations
- Not suitable for active or moving cracks — filling an active crack will result in re-cracking
- Not suitable for wet area applications — use a sanitary silicone or PU sealant in wet areas
- Not a structural repair — do not use on structural cracks or cracks with differential displacement
PROCUREMENT SOURCES
Confirm suitability with the current manufacturer TDS before specifying or applying.
System Description
Sikaflex-111 Stick & Seal is a 1-component moisture-curing flexible adhesive-sealant based on silane-terminated polymer technology used for movement joints and crack repair in internal render and masonry surfaces. Suitable for wider internal cracks and movement joints where a flexible acrylic filler does not provide sufficient movement accommodation — accommodates ongoing thermal movement and is overpaintable once cured with compatible interior paint systems. Bonds to a wide variety of substrates including concrete, masonry, ceramic, wood, metal and plastics without primer in most cases. For active movement joints, install backer rod to control sealant depth and prevent three-sided adhesion. Prime joint faces per Sika TDS on porous substrates (Sika Primer-3 N is referenced). Not suitable for continuously wet conditions without specific Sika approval. Confirm current product designation and TDS with Sika Australia before specifying — product names are subject to periodic revision.
Technical Properties
- 1-part moisture-curing silane-terminated polymer (STP) — no site mixing required, reduces application error
- Overpaintable after full cure with compatible interior paint systems — confirm with Sika TDS
- Good elongation and movement accommodation for internal movement joints and wider cracks in render and masonry
Limitations
- Not suitable for active cracks without backer rod — three-sided adhesion prevents proper sealant movement
- Not a structural repair — do not use on structural cracks or cracks with differential displacement (step cracking)
- Do not apply to damp or contaminated joint faces — primer mandatory on porous substrates per Sika TDS
PROCUREMENT SOURCES
Confirm suitability with the current manufacturer TDS before specifying or applying.
System Comparison
Side-by-side technical comparison of internal crack repair products. Confirm current product specifications with manufacturer TDS.
| Product | Brand | Chemistry | Paintable | Crack width | Movement | Primary use |
|---|---|---|---|---|---|---|
| No More Gaps Interior Multipurpose | Selleys | Acrylic | Yes | Hairline to ~5mm | Minor thermal | Plasterboard and render hairline cracks — cornices and butt joints |
| Sikaflex-111 Stick & Seal | Sika | Silane-terminated polymer (1C) | Yes (confirm TDS) | Fine to wide — with backer rod | Thermal + live load | Movement joints and wider cracks in render and masonry — internal |
Brand Equivalents
Internal crack repair equivalents across brands active in Australian Class 2 strata remediation.
| System type | Selleys | Sika |
|---|---|---|
| Acrylic gap filler | No More Gaps Interior Multipurpose | — |
| Flexible STP sealant (movement joints) | — | Sikaflex-111 Stick & Seal |
Structural cracks require engineering assessment
- Cracks wider than 3mm or with differential displacement (step) should be assessed by a structural engineer before filling — crack filling a structural crack conceals a defect and is not a repair
- Active cracks (still moving) will re-crack through filler — movement joint sealant with backer rod is required for joints with ongoing thermal or live load movement
- Never fill a crack in a wet area without confirming the waterproofing membrane integrity below — filling over a failed waterproof junction hides a defect and allows water ingress to continue
Disclaimer
Information is general only. Structural or active cracks must be assessed by a structural engineer before crack filling. Crack filling is appropriate for dormant cosmetic cracks only. Product selection must be confirmed against crack width, substrate, and the current manufacturer technical data sheet. Do not rely on this reference as a substitute for professional advice.